1. Field of the Invention
This invention relates to an apparatus and a method for filtering air in a predetermined area, wherein the apparatus works in combination with an air-conditioning unit and an environmental monitoring unit. More specifically, the present invention is directed to a method and device for filtering contaminants from a contaminated area in an efficient and safe manner, which is used with an air-conditioning unit or an environmental monitoring system.
2. Description of the Related Art
It is often desirable to filter the air in a closed room due to the presence of contaminants. These contaminants may originate from equipments leaks, chemical spills or any other circumstance whereby contaminants have entered the air. An example of such a setting is a nuclear power facility, where it is desirable to reduce or eliminate the spread of radioactive particles so that the amount of radioactive particles that workers are exposed to is well below preestablished limits.
In the nuclear industry, furthermore, it is desirable to reduce heat stress upon individuals who work in radioactive contamination areas must be dressed in anti-contamination clothing. One proposed method for reducing heat stress is to provide air conditioning to the work area. However, this presents a problem because the work area must be kept under a negative pressure. Otherwise, radioactive contamination would migrate from the work zone. If air is blown directly into a contaminated work area, the pressure in the area will become positive and the radioactive contamination will migrate to areas outside the work zone.
It has also been suggested that an air conditioning system be used in a conventional manner. That is, supply cool air to the work area and remove warm air. However, such a method and apparatus has also proven ineffective, as contamination is removed with the warm air, thus contaminating the air conditioning unit.
A solution to this problem has been proposed. That is, it has been proposed to use a HEPA filtered ventilation system in combination with the air conditioner. A typical ventilation unit has a filter unit and a fan. A gauge is mounted across the filter unit for measuring the differential pressure. The filter unit contains two filters, a prefilter and a HEPA (high efficiency particulate air) filter. Most known units can also be supplied with a carbon filter. All ventilation units, regardless of capacity, are operated in the same manner. Flexible ducting is coupled to the intake of the filter housing and directed to the area that requires ventilation. The exhaust fan is then started and the unit is run until the room has been completely ventilated or the pressure gauge indicates a high reading, suggesting that the filter(s) need changing.
Connecting such a HEPA ventilation unit to an air conditioning unit has several problems. No visual method is known for determining the amount of air flowing through the system. To determine the system air flow requires that velocity measurements be taken, which involves placing a measuring instrument into a potentially contaminated piece of duct work and contaminating the instrument.
As the ventilation system operates, the filters in the filter unit begin to filter out the contaminant in the air. More contaminants are filtered out as the system continues to run. The contaminant filtered out of the air and trapped on the filters cause the static pressure across the filters to increase, which increases the static pressure facing the exhaust fan. The total flow through the ventilation system is thus decreased. The higher the static pressure existing across the filters, the less the air flow. Therefore, it is necessary to perform continuous measurements of the velocity to determine the air flow. However, air conditioning systems are designed to have a certain amount of air passing over the evaporator coils. If too much air passes over the coils, moisture is carried off the coils into the working area. If too little air passes over the coils, the coils will freeze and prevent any air from passing over them.
Another problem faced by the nuclear power industry is the determination of how much radioactive contaminant has been released into the environment. The amount of released contaminant is strictly regulated and large fines can be imposed for nuclear plants that have unregulated releases. All nuclear stations have main ventilation systems equipped with environmental monitors that determine the amount of contamination released into the environment. The main ventilation systems monitor most of the large interconnected buildings, but do not monitor the smaller remote buildings or temporary buildings where work is often performed on contaminated materials. For these areas, nuclear stations will normally perform grab sampling every four hours or so to determine the amount of radioactive material released to the environment. In order to determine properly the amount of radioactive contaminants released to the environment, isokinetic air sampling should be performed. This means that the air being monitored should move at the same velocity at the sampling probe and throughout the exhaust duct of the ventilation unit. However, prior ventilation systems that have been used do not move a constant volume of air, and thus the displaced air moves at a changing velocity.